Wireless devices have been in use for many years for enabling mobile communication of voice and data. Such devices can include mobile phones and wireless enabled personal digital assistants (PDA's) for example. FIG. 1 is a generic block diagram of the core components of such wireless devices. The wireless core 10 includes a baseband processor 12 for controlling application specific functions of the wireless device and for providing and receiving voice or data signals to a radio frequency (RF) transceiver chip 14. The RF transceiver chip 14 is responsible for frequency up-conversion of transmission signals, and frequency down-conversion of received signals. RF transceiver chip 14 includes a receiver core 16 connected to an antenna 18 for receiving transmitted signals from a base station or another mobile device, and a transmitter core 20 for transmitting signals through the antenna 18 via a gain circuit 22. Those of skill in the art should understand that FIG. 1 is a simplified block diagram, and can include other functional blocks that may be necessary to enable proper operation or functionality.
FIG. 2 is a more detailed circuit schematic of the transmitter core 20 of wireless core 10 shown in FIG. 1. The transmit core 20 includes an up conversion, or mixer circuit 30, a variable gain circuit 32, an automatic gain control (AGC) circuit 34, output pin 36, SAW filter 38, and driver circuit 40. Those of skill in the art will understand that there may be additional components of the transmit core 20 that are not shown in FIG. 2, but are required to enable proper operation of the circuits. Up conversion circuit 30 receives a baseband signal IN to be transmitted, and up converts the baseband signal to a desired transmission frequency z. The unconverted signal is amplified by variable gain circuit 32 in response to signal VGAIN. VGAIN is generated by AGC circuit 34 in response to control voltage VCONT. AGC circuit 34 is responsible for sensing at least one parameter of the device, such as temperature, process and voltage for example, and adjusting the control signal VGAIN to maintain a substantially linear relationship between VCONT and the gain from variable gain circuit 32. The output of variable gain circuit 32 is coupled to off chip SAW filter 38 via output pin 36, for filtering noise of the signal to be transmitted. The filtered signal is then driven by driver circuit 40 to the antenna 18 of the wireless device.
An important function of wireless core 10 is to control transmission signal gain in response to base station requests. Typically, the base station in communication with the wireless device will instruct the wireless device to increase the gain for transmission, since the previously transmitted signals may have been detected as being sub-optimal. Those of skill in the art will understand that the request from the base station is embedded within the communication signal being transmitted to the wireless device. This increase can be specified as being a 10 dB increase, for example. Alternately, the base station can instruct the wireless device to reduce gain, in order to conserve battery power of the wireless device while maintaining optimal performance. To adjust the gain provided by variable gain circuit 32, baseband processor 12 will generate an analog input control voltage signal VCONT for controlling variable gain circuit 32 to provide the desired gain.
As previously mentioned, the relationship between the desired gain and the voltage level of VCONT should be substantially linear, and many standards presently in use specify a close to linear relationship between VCONT and gain. Such standards include EDGE and WCDMA communications standards for example.
Most radio frequency (RF) devices, which typically include gain circuits, are manufactured using SiGe, GaAs, or other heterojunction technologies. Those of skill in the art will understand the advantages provided by SiGe and GaAs devices. GaAs devices have higher electron mobility, run on low power, and generate less noise than traditional CMOS devices, while SiGe heterojunction devices have good forward gain and low reverse gain characteristics, which translate into low current and high frequency performance than typically available from homojunction or traditional bipolar transistors. Gain circuits fabricated with such technologies generally exhibit a substantially linear relationship between gain and VCONT. However, such manufacturing technologies are relatively new, very complex, and hence expensive. Consequently, the costs for manufacturing these RF devices can be prohibitive. Complementary Metal Oxide Semiconductor (CMOS) technology on the other hand, is a very mature and inexpensive fabrication process for the production of semiconductor devices.
FIG. 3 is an example circuit schematic of the up conversion circuit 30 shown in FIG. 2, implemented in CMOS technology. It is noted that the circuit of FIG. 3 is configured for differential signals, and the circuit of FIG. 2 is a simplified schematic representing the differential signal configuration. Up conversion circuit 30 includes dual differential pairs, each for driving a respective phase of the unconverted signal. The first differential pair includes n-channel transistors 50, 52 and 54, where transistor 50 is coupled to VDD through common load resistor R1, and to VSS through transistor 54. Transistor 52 is coupled to VDD through common load resistor R2, and to VSS through transistor 54. The gates of transistors 50 and 52 receive complementary up conversion frequency signals z and z* respectively, while the gate of transistor 54 receives one phase of input baseband signal IN.
The second differential pair includes n-channel transistors 56, 58 and 60, where transistor 58 is coupled to VDD through common load resistor R2, and to VSS through transistor 60. Transistor 56 is coupled to VDD through common load resistor R1, and to VSS through transistor 60. The gates of transistors 56 and 58 receive complementary up conversion frequency signals z* and z respectively, while the gate of transistor 60 receives the opposite phase of the input baseband signal, labelled IN*. The operation of up conversion circuit 30 is well known to those of skill in the art. The circuit multiplies the baseband input signal IN/IN* with the up conversion frequency z/z* to provide corresponding output signals OUT and OUT*. The first differential pair drives output signal OUT while the second differential pair drives opposite phase output signal OUT*.
FIG. 4 is an example circuit schematic of the variable gain circuit 32 shown in FIG. 2, implemented in CMOS technology. It is noted that the circuit of FIG. 4 is configured for differential signals, and the circuit of FIG. 2 is a simplified schematic representing the differential signal configuration. The variable gain circuit includes two differential pair circuits, similar to the ones shown in FIG. 3. The first differential pair includes n-channel transistors 70, 72 and 74, where transistor 70 is coupled directly to VDD, and to VSS through transistor 74. Transistor 72 is coupled to VDD through load resistor R3, and to VSS through transistor 74. The gates of transistors 70 and 72 receive differential gain control voltage V_GAIN− and V_GAIN+ respectively, while the gate of transistor 74 receives signal OUT* from up conversion circuit 30 of FIG. 3. It is noted that OUT* in FIG. 3 can be coupled as in FIG. 4.
The second differential pair includes n-channel transistors 76, 78 and 80, where transistor 76 is coupled directly to VDD, and to VSS through transistor 80. Transistor 78 is coupled to VDD through load resistor R4, and to VSS through transistor 80. The gates of transistors 76 and 78 receive differential gain control voltage V_GAIN− and V_GAIN+ respectively, while the gate of transistor 80 receives signal OUT from up conversion circuit 30 of FIG. 3. It is noted that OUT in FIG. 3 can be coupled as in FIG. 4.
The operation of variable gain circuit 32 is well known to those of skill in the art. Maximum gain of signals OUT* and OUT is obtained when V_GAIN+ is at a maximum voltage level, and minimum gain of signals OUT* and OUT is obtained when V_GAIN+ is at a minimum voltage level. The first differential pair drives output signal Vpin+ from a corresponding output pad, while the second differential pair drives opposite phase output signal Vpin− from another corresponding output pad. These output pads correspond to output pad 36 shown in FIG. 2.
Ideally, baseband signal IN/IN* is unconverted and amplified linearly and with minimum noise as output signals Vpin+/Vpin− such that they can meet the minimum requirements for one or more of the previously mentioned communication standards. Unfortunately, the CMOS variable gain circuit 32 does not exhibit a substantially linear characteristic between gain and the input control voltage VCONT, which is equal to V_GAIN+-V_GAIN−. In fact, CMOS transistors in general do not exhibit substantially linear voltage-current characteristics. It is for this reason that AGC circuit 34 must be included to compensate for any introduced signal non-linearities due to the inherent non-linearity of CMOS transistors. Those of skill in the art will further understand that the non-linearity of CMOS transistors can be further complicated by PVT (process, voltage, temperature) variations. Those of skill in the art will appreciate that any one of these variants can affect the operating characteristics of transistor devices, and ultimately, the gain characteristics of the circuit.
Of the PVT variants described, process and voltage are generally static variants that typically do not change during operation of the wireless device. Temperature on the other hand, can change significantly during normal operation of the wireless device. FIG. 5 is a graph illustrating example gain responses of a variable gain circuit as a function of control voltage VCONT for different operating temperatures. Curves 90, 92 and 94 are the gain-VCONT relationships at 85, 22.5 and −40 degrees Celcius, respectively. While all three curves are substantially linear, the variance with temperature, and therefore overall linearity of the variable gain circuit, is not achieved since the amount of gain can vary by as much as 20 dB for a given VCONT value.
There are various techniques and corresponding gain control circuit implementations for AGC 34 that are known in the art for ensuring that actual gain of the variable gain circuit 32 follows a linear relationship with the control voltage VCONT. Commonly owned U.S. application Ser. No. 11/092,566, which is incorporated by reference, discloses a digital system for gain control by monitoring any one of temperature, supply voltage and process parameters, and generating a corresponding compensated gain control voltage for a variable gain circuit. Persons skilled in the art will understand that the above-described AGC system represents one possible technique for correcting/compensating for the inherent non-linear properties of CMOS circuits. Other possible AGC systems can include feedback systems or systems that employ reference circuits.
While the previously discussed AGC circuit effectively establishes a substantially linear relationship between the gain control signal VCONT and the actual gain from variable gain circuit 32, the effective range is limited to about 40 dB. This is sufficient for standards such as GSM and EDGE, but standards such as WCDMA require a higher minimum range of about 85 dB. Therefore, any wireless core employing such an AGC circuit will not meet the WCDMA standard. This is mainly due to the inherent non-linear behaviour of CMOS transistors operating in a saturation mode.
Those of skill in the art will understand that CMOS circuits, such as the variable gain circuit 32, are typically operated in the saturation mode. Although the saturated transistor operates as an ideal current source, it will have a non-linear relationship between its drain current (Id) and its gate-source voltage (Vgs). Furthermore, to keep the transistor operating in the saturation mode, the transistor drain-source voltage (Vds) must be greater than or equal to Vgs-Vt, where Vt is the threshold voltage of the transistor. Hence voltage headroom is reduced, which can lead to clipping of the input signal.
In addition to standards specifying gain characteristics, there are standards governing the maximum amount of allowable noise in the resulting unconverted and amplified signal. Unfortunately, the circuit of FIG. 2 requires the addition of SAW filter 38 to remove unwanted noise from the signal that is generated within up conversion circuit 30, variable gain circuit 32, and even from AGC circuit 34.
With reference to the circuit schematics of FIG. 3 and FIG. 4, noise is introduced in each current to voltage and voltage to current conversion stage of the circuits. Starting in the up conversion circuit 30 of FIG. 3, signals IN/IN* are voltage signals switching transistors 54 and 60 on and off to generate currents through transistors 50, 52, 56 and 58. These currents are then converted to voltage signals OUT/OUT* and provided to the variable gain circuit 34. At the variable gain circuit 32, voltage signals OUT/OUT* switch transistors 74 and 80 on and off to generate signal currents through transistors 72 and 78. These currents are then converted to voltage signals Vpin+/Vpin−. Each voltage to current and current to voltage conversion stage will introduce noise into the resulting output signals Vpin+/Vpin−. In addition to noise, the numerous voltage to current and current to voltage conversion stages will consume current and therefore waste power, which is a limited resource in mobile wireless devices.
CMOS transmitter circuits, especially up conversion circuits, variable gain circuits, and automatic gain circuits are less costly to manufacture than their more exotic bipolar counterparts. While the inherent non-linearity of CMOS variable gain circuits can be compensated/corrected with existing circuits, they are limited to a 40 dB range that is insufficient for WCDMA standards. Furthermore, the noise added by the mixer circuit and the variable gain circuit necessitates an off-chip SAW filter discrete component, potentially increasing the wireless device form factor and cost due to the additional device.
It is, therefore, desirable to provide a CMOS transmitter with a gain system for providing a high range of gain and linear operation, while minimizing noise.